Retinol is an essential cofactor in growth regulation of lymphocytes and other cells. Intracellularly, retinol is converted into 14 hydroxy-retro- retinol (14HRR), the first naturally occurring retro-retinoid to be discovered. 14HRR substitutes for retinol at 100-fold lower concentration and therefore qualifies as a second messenger molecule. The biosynthesis of 14HRR in lymphocytes involves as an intermediate 13,14 dihydroxy retinol (DHR), also a new retinol discovered by us. This conclusion is based on kinetic analyses indicating that DHR synthesis precedes that of 14HRR, and furthermore on biochemical evidence establishing a direct precursor-product relationship. The rate of retinol consumption is dependent on the state of activation, being low in resting T cells but accelerating approxiately 10-fold during activation of T cells. The increase in retinol consumption is transient and is parallel by synthesis of DHR and 14HRR. These data strongly imply regulation of retinoid metabolism and form another argument for the view that 14HRR is a second messenger molecule. Studies on activation requirements in T cells have shown that retinol or 14HRR needs to be present during the first day of activation. When blast transformation has occurred and T cells proliferate, they become independent of retinol or 14HRR. The early dependence on retinol cannot be overcome by lymphokines (IL-1, 1-2, IL-4, IL-6 or TNFalpha) but in the presence of IFN-gamma substantial T cell activation can also occur in the absence of retinoids. While 14-HRR-responsive genes have not been identified, we have noted small but reproducible influences on the activation of immediate early genes. Whereas it is unlikely that the dramatic difference in cellular responses elicited by retinol can be explained on the basis of immediate early gene regulation, the observed transcriptional enhancement implies a nuclear site of action. A search for the putative nuclear receptor is in progress.